Individual mangrove trees provide alternative reef fish habitat on backreefs

Mangrove trees occur in a variety of geomorphic and sedimentary settings. Yet, studies investigating their role as habitat providers often focus on the most common biophysical types, such as deltaic, estuarine, open coast or lagoonal mangroves on soft sediments, disregarding less typical environments. Here, we investigated the influence of individual mangrove trees growing on a consolidated backreef system (Laucala Bay, Fiji) on habitat use by reef fishes. Combining field surveys and an experiment, we quantified the extent to which reef mangrove trees serve as habitat for solitary or shoaling reef fishes. Using mangrove tree mimics, we disentangled effects attributable to the physical structure of trees from those related to their bio-chemical properties. We found that fish numbers were 3.7 times higher within close proximity to the mangrove trees than at control sites and correlated significantly with root system perimeter. The roots of larger trees sheltered aggregations of juveniles and adults at incoming and high tides. Mangrove trees and mimics attracted fishes alike. We show that mangrove trees on backreefs provide habitat for shoaling and adult reef fishes in addition to serving as nursery areas, an ecosystem service otherwise lacking on backreef areas with low structural complexity.


Mangrove trees on backreefs
iii S4: Literature overview Throughout the twentieth century, various occurrences of mangroves in association with coral reefs (sometimes referred to as "reef mangroves" or "oceanic mangroves") have been reported and investigated (Fig. S4).
Fig. S4 Bathymetric profile of a tropical lagoon with a dense coastal mangrove forest near the shore (left) and reef mangroves situated on a reef platform offshore (right).Modified after Rützler and Feller (1996).
The geographic range of these accounts spans around the subtropical belt, from the Great Barrier Reef to the Caribbean and the Red Sea (Por & Dor, 1975;Rützler & Feller, 1996;Stoddart, 1980).These reports are mainly based on observations, repetitive mapping, and aerial photography of the areas.Researchers studied the changes in these mangrove-reef habitats over time, leading to several hypotheses about their origin and mechanisms of establishment of mangrove covered reef flats and islands (Spender, 1930;Steers & Lofthouse, 1940;Stoddart, 1978;Woodroffe, 2018).Some hypotheses imply that reefs undergo successional stages including a 'mangrove stage' in mature reefs, in which mangroves settle on the reef flat, slowing water movement and trapping sediment, ultimately leading to the formation of land (Fairbridge & Teichert, 1948;Spender, 1930;Stoddart, 1980).Others argue that the occurrences of mangroves on reef flats are opportunistic and a response to local substrate and wave energy conditions (Steers & Lofthouse, 1940;Woodroffe, 2018).Most of the studies are descriptive, though a few investigate particular ecological aspects such as biodiversity or ecological roles of mangrove species (Rogers, 2017;Stearn, 1958).Notably, the term 'Coexisting Mangrove-Coral Habitat' (CMC) has been used in recent studies to classify corals growing on mangrove roots or under mangrove canopy (Stewart et al. 2022).This constellation provides coral with a certain level of shelter from environmental stressors and has increasingly been studied in light of climate change exacerbating such stressors (Rogers and Herlan, 2012;Yates et al., 2014;Scavo Lord et al., 2020).
An overview of the body of literature on mangroves and reef associations is provided in Table S4, disentangling the different mangrove/coral interaction contexts, and identifying those that could be considered 'reef mangroves' in the context of this paper.None of the studies identified in this review focus on the role of individual mangrove trees, and none of the studies on reef mangroves are investigating their role as reef fish habitat.

Mangrove trees on backreefs iv
Mean and range (min-max)of total body length (TL) for the taxa observed in fish counts.Estimated TL at maturity recovered from literature is also provided.
FAO.The Living Marine Resources of the Western Central Pacific -FAO species identification guide for fishery purposes.Volume 5 B. Carpenter KE, Niem VH, editors.Food and Agriculture organization of the United Nations; 2001.(2) Froese R, Pauly D. FishBase [Internet].[cited 2023 Oct 2].Available from: https://www.fishbase.orgS3: Taxa composition of observed shoaling fish, maximal & mean shoal sizes (min shoal size was set to 20 individuals), number of shoals per taxa, and total sum of observed shoaling individuals of each taxa.

Table S4 :
Review of literature on mangroves and coral reefs in direct proximity or direct contact with each other, comparing location, aspect studied, scale at which the system was studied, and the particular mangrove/coral interaction context.Any mangrove descriptions matching 'reef mangroves' as defined in this paper are highlighted in bold/italic and marked with an asterisk in the 'Mangrove/Coral Interaction Context' column.